If your Mbuna need a fairly veggie-based diet (OSI spirulina flake, Wardley's or a similar brand), the best choices are:
-Synodontis petricola (this one's quite rare in the trade and often expensive wildcaught)
-Synodontis "petricola dwarf" (this is the one commonly sold as 'petricola') from Zambia
-Synodontis sp "Giant petricola" from Tanzania
-Synodontis "polli White" from Zambia (commonly sold as polli)
All do best in shoals (5-7 fish), and 1 petricola-species can be combined with the "polli White" as well, but best not combine petricola-species. even if you don't intend to breed with those, fierce fights can occur between the dominant individuals of the species.
With -example- 5 "polli Whites" and 7 "petricola dwarf" in your Mbuna-tank, you'll have plenty visible catfish
NONE of these are broodparasites btw.
Riverines wich can handle both the Mbuna, the veggie-food and the higher pH include S eupterus and S nigrita.
Other Synodontis-species (Lake or riverine) need a more varied foodregime (frozen crustaceans, flakes or granules with more protein). if your Mbuna can handle that, the list gets a bit longer!.
Plecs:
THE plec for Rifttanks is the Bristlenose, because it doesn't grow to monstertruck-size
and it also keeps cleaning algae to the very end. BN's reach 4" or so; the captive-bred brown/albino/L144 are most suitable.
I wouldn't put a "veiltail" or "longfin"bristlenose in with Mbuna
for obvious reasons.
If a bigger Plec is more to your liking, 2 are suitable: the Gibby (Pterygopichthys gibbiceps) wich is very pretty but reaches the same size as the Common Plec, and the L001 (Pterygoplichthys joselimaianus) wich usually stays a bit smaller at 10" or so. that one has the coloration of an "inverted" Gibby
instead of brown spots in an orange network it has orange spots in a brown network. very goodlooking fish.
ALL Plecs/Bristlenoses need a cave where they will not be buggered by the Mbuna, or they can loose their eyes, finparts and die. it happens quite often.